1. Field of the Invention
This invention relates to forming polyurethane articles in a spray forming process.
2. Background Art
Over time, polyurethane materials have been developed and implemented in processes used in manufacturing articles. For example, in the manufacture of automotive interior components, it has been proposed to form such components by spraying a gellified polyurethane material into a mold to form a skin that is later placed in a polyurethane foam forming mold and filled with structural polyurethane foam to form at least part of a finished part. One such gellified polyurethane foam skin, as proposed in U.S. Pat. No. 5,071,683, is formed by combining a mixture of polyurethane reactants that is mixed under pressure in a spray gun and then sprayed as a mixture by a single nozzle in the form of a film and/or drops.
In the '683 patent, a mixing chamber in the spray gun has an input for polyol and isocyanate components in which the polyol and the isocyanate are mixed in the mixing chamber with a high speed counter-current that is then dispensed through a nozzle. The nozzle is designed to form a truncated cone shaped spray that breaks apart into droplets prior to being deposited on a surface. This approach to form a gellified polyurethane skin suffers from the disadvantage of requiring frequent purging of the mix gun. Purging the reactants wastes expensive polymer materials. The system also requires solvent flushing because the polyol and the isocyanate mix within the mix head or nozzle causing the system to become clogged or blocked.
With an internal mixing system, the polyol and the isocyanate reactants are mixed at high pressure requiring the system to be designed to withstand high pressures to provide a good internal mix of the polyol and isocyanate. This design can result in nozzles being partially or completely plugged. In addition, high pressure operation results in increased wear and tear on the spray line equipment. High pressure pumps required for an internal mixing system are considerably more costly than systolic, diaphragm, or gear pumps that may be used in lower pressure systems. Spray tips based on this concept is limited in amount of output that can be delivered and, therefore, may lead to increased cycle times.
In summary, prior art internal mix systems are more costly due to raw material waste, waste of solvents for cleaning spray equipment, and increased machinery costs due to high pressure system components. Prior art internal mixing systems also require longer cycle times due to the time required to purge material from the system and flush the system with solvent.
These and other problems are addressed by applicant's invention as summarized below.